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Window Terminology


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General Window Terminology
Airspace
The space in the cavity between two panes of glass in an insulating glass unit.
Arch Top
A window that incorporates curves, usually half or quarter circle. View Image
Argon Gas
An inert, nontoxic gas used to fill the airspace between insulating panes of glass to improve energy efficiency.
Awning Window
A window unit, similar to a casement, where the bottom of the sash swings outward. View Image
Bay Window
An arrangement of three windows, usually made up of a large center unit and two flanking units at 30 or 45 degree angles to the wall. View Image
Bow Window
A combination of four or more window units in a radial or bow formation. View Image
Bottom Rail
The bottom horizontal member of a window sash.
Brickmold
An external trim piece that frames windows and doors.
Casement
A window with a side hinged sash that opens outward, to the left or right. View Image
Casing
Molding or framing applied to the framework of doors and windows. View Image
Conduction
Heat transfer through a solid material, such as glass, through direct contact. Heat flows from the area of higher temperature to one of lower temperature.
Convection
The flow of heat that occurs in a fluid such as air as warm air rises and cool air sinks.
Direct Set
A stationary window with no sash, where the glass is set directly into the frame.
Divided Lite
A window with a number of small glass panes held in place by muntins.
Egress Window
A window large enough to allow for an emergency exit. Most building codes require a minimum opening of 5.7 square feet when the sash is open, but egress requirements are mandated by local building codes.
Extension Jamb
Wood pieces attached and extending from a window or door frame to accommodate different wall thickness.
Extrusion
A form produced by forcing heated material through a die.
Fenestration
A window and its associated interior or exterior elements.
Flanker
A window joined to the side of another window or door.
Frame
The area in which the window sash or door panels are mounted. Frame components include the head jamb, sill, side jambs and blind stop.
French Door
A frame with two operating door panels hinged on the side and swinging open from the center of the unit. An astragal holds one door panel stationary while the other panel operates.
Gas Filling
An insulating gas, usually argon, placed between window panes to reduce the U-factor by suppressing conduction and convection.
Glazing
Glass in a window or door, or the process of its installation.
Glazing Stop
The part of the sash holding the glass in place.
Grid
A grid installed on or between the glass lites that visually divides a window into panes. View Image
Insulated Glass Unit (IG)
Two pieces of glass separated by a spacer and hermetically sealed to form a single glazed unit.
Lite
A pane of glass within a window or, in some cases, an entire glass unit.
Low E Glass
A special glass type with a microscopically thin, metallic silver oxide bonded to one side of the glass that reduces heat transference, lowering home heating and cooling costs.
Marginal Lites
A grid pattern of rectangular perimeter intersection aligned 4 inches from the edge of the glass. View Image
Mull Casing
A casing covering the joint between two windows mulled together.
Mulled
Joining two windows or doors together, side by side.
Mullion
A wood or metal piece used to structurally join together two or more windows.
Muntin
The bar, vertical or horizontal, used to create the grille patterns in or on the glass. Also referred to as grids or grilles.
Nailing Fin
A strip of material used to attach windows to a house.
NFRC
National Fenestration Rating Council.
Obscure Glass
A translucent but not transparent glass. There are many types of obscure glasses to choose from.
Pane
See LITE.
Picture Window
A window with no operating hardware. View Image
Queen Anne Style Grids
A grid pattern of rectangular perimeter intersection aligned 4 inches from the edge of the glass. View Image
R-Value
A measure of the resistance to heat flow. A high R-value indicates a window has a greater resistance to heat flow and a higher insulating value. The reciprocal of U-VALUE.
Rail
The top and bottom horizontal pieces of a window sash.
Rough Opening (RO)
The framed opening in a wall that will house the windows. Usually a half inch in both directions, or 1/4" all the way around, is allowed for insulating and squaring the unit.
Sash
The frame composed of stiles and rails that surrounds and holds the glass.
Segmented
A series of straight pieces joined together to create the appearance of a radius.
Side Jamb
The vertical pieces of a window or door frame.
Sidelite
A fixed unit installed on one or both sides of a doorframe, for decoration and light.
Sill
The bottom, horizontal section of the window frame.
Simulated Divided Lite (SDL)
A method of attaching muntin between or on the glass surface to simulate smaller individual glass panes.
Single Hung
Similar to a double hung, except that the top sash is fixed and inoperable. View Image
Slider
A window where the sashes move horizontally in grooves created by frame members.
Spacer
Material placed between the two pieces of glass in an IG.
Stationary
A door or window panel that does not open or move.
Stop
The molding applied to the window frame that holds, positions, or separates window parts.
Surround
A frame of wood enclosing the perimeter of a removable grille.
Tempered Glass
Glass that has been through a special heat-treated tempering process that increases its strength. When shattered, the glass breaks into small pieces to reduce injury.
Thermal Performance
The ability of a window to act as a barrier to the transfer of heat.
Transom
A small window above a door or another window.
True Divided Lite (TDL)
A window in which individual panes of glass are placed into a sash, each pane separated using muntins.
U-Value
A measure of heat transmission through a window. A lower value indicates less heat transference and better energy efficiency. The reciprocal of R-VALUE.
Ultraviolet Light
Invisible rays produced by the sun that can cause fading of furniture fabrics, carpets and paint finishes.
Unit Dimension
A dimension that is equal to the frame size. The unit dimension is measured from the outermost edges of the window's main frame.
Vinyl
Polyvinyl chloride material, either rigid or flexible, used for window frames.
Weatherstripping
A material that creates tight seal between a sash frame to prevent water and air infiltration.
Z-Bar
The outer frame or flange on a replacement window that covers the perimeter of the opening. View Image


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